Pin debian:wheezy as base docker image.
Add postgresql-client and remove mysql packages.
Copy and install python reqs first.

Take better advantage of docker caching by copying and installing python
packages before copying the rest of the code. This way if requiments.txt
or peep.py doesn't change docker will use it's cached image and avoid
re-installing everything on code changes.
Add .git in .dockerignore.

We don't need to send git directory as part of the git context. Not only
it increases the size of the resulting container, it also messes up with
docker caching.
Use postgres:9.3.
Merge pull request #23 from glogiotatidis/tag-docker-debian

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